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Today I inspected the drain and trap under the kitchen sink. I haven't really had any specific problem but I guess the drain could be faster. Sometimes I start the disposer to make it drain quicker. I have spotted some sort of grease leaking out of the trap and I will clean that up but my question is - is this an optimal drain plumbing? Something seems off in this installation but maybe it's just my lack of experience.

On the right side, difficult to see in these particular pictures, there is a copper pipe section that goes down to the sewer and also up to a vent on the roof.

Please, forget the section of drywall I removed. I just wanted to see the hot and cold water pipes as I'm also trying to fix a water hammer issue, but that is another story.

enter image description here enter image description here

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    The only issue I see is that the pipes to the trap stagger in the wrong order. This means that there is standing water in the horizontal section from the T to the trap. But I'm not sure how much of an issue that actually is. – ratchet freak Mar 26 '19 at 10:17
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    I see another issue unrelated to plumbing. The use of the flat Romex power cable as an extension cord is not specified for that type of usage. That type of cable is supposed to be concealed inside walls, raceway with cover or conduit. The end where it connects into the electrical supply should also be a permanent connection inside the electrical box. Note that there are more appropriate types of appliance connection cables that can be used to plug into the outlet. They will look a lot more like the cord used to take power to the disposer. – Michael Karas♦ Mar 26 '19 at 13:31
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    @ratchetfreak I'm going to say that could be a big reason for a slow drain. There's going to be standing water in that whole section coming off the T, and standing water leads to gunk deposit. – JPhi1618 Mar 26 '19 at 15:08
  • I can see how when the water is forced uphill after the trap could be a bit of a bottleneck. – stevieb Mar 26 '19 at 15:23
  • Thanks can you explain a little better the "stagger in the wrong order"? I am not quite sure what that means, even if I understand the fact there is a horizontal section with standing water. As a side question, should each half of the sink have their own trap or one is just fine? – Alessio Sangalli Mar 26 '19 at 15:33
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This is what you need to do.

enter image description here

What you have is essentially a very long and deep trap seal. For a trap to function properly you need a seal (height of stored water) to be between 2 and 4". So from the bottom of the exit pipe to the bottom of the trap should be between 2 and 4". The reservoir of water should only be contained in the trap.

You want the drain to slope the entire way, 1/4" per foot is the only acceptable slope. This allows the waste water to wash the pipe. It swishies back and forth at 1/4" slope. More slope and the waste shoots down the center building up debris on the wall of the pipe. Less slope and it doesn't flow.

Most people don't know this for some reason but on a torpedo level half the bubble is 1/2" slope per foot, 1/4 bubble is 1/4" per foot.

Some levels have multiple indicators to aid in the measurement. In this case 1/8th and 1/4" respectively.

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www.swansontoolco.com

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  • As the rules say, I cannot just "thank you" in this comment. But thank you. I did not know about those signs on a level. Mine does not have them anyway. I typically just use my phone with a "bubble" app that I find quite accurate. Thank you for pointing out the slope is important and that it must not be too much, I really did not know that, I thought the steeper the better. – Alessio Sangalli Mar 27 '19 at 19:35
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As mentioned in the comment by ratchet freak, the drain pipe doesn't lead in a downward direction the whole way. Each horizontal piece should be staggered so that it ends up lower than the last piece.

The horizontal piece that goes into the trap on the left is lower than the piece coming out of the trap on the right. It should be the other way around. What that means is that any part of the pipe that is under the level of the drain in the wall will always be "under water". Standing water gives sediment and grease a chance to collect and turn into a sludge in the pipe that restricts the flow of water.

It's easy to check - just take apart some of the joints so you can see inside the horizontal sections. If they have a lot of collected junk, then that's a problem. If they are clean, then you're lucky and the restriction could be somewhere else.

Having one trap is correct. Since your disposal is on the lower side of the sink it makes running a proper downward slope a little more complicated, but it is still possible. Again, checking the buildup in the pipes will tell you more about the problem.

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  • Thank you, so it was meant as a "vertical" stagger, now I understand it. Let me put it this way: I have to remove the drain to work on the pipes and drywall behind it. How do you recommend I change the current drain system when I put it back together to be "right"? Is the P-trap currently installed "backwards"? I spent several minutes trying to visualize how I should fix this but I am still not sure. – Alessio Sangalli Mar 26 '19 at 16:24
  • The trap is backwards. If you re-do the drain, the left drain will need to go horizontal sooner, probably the same height as the disposal tailpiece, maybe a bit lower. Then you will have a T on the horizontal (tilted up just slightly) that the disposal can feed into with an elbow. The end result should be getting to the trap higher than where the drain enters the wall. – JPhi1618 Mar 26 '19 at 16:33
  • OK maybe I understand :D I will try some modifications to keep the "upstream" part of the drain physically higher. I think I have "visualized" a way. – Alessio Sangalli Mar 26 '19 at 17:30
  • Yea, it's a 3-dimensional system, so it can be hard to describe and visualize. The main idea is trying to keep everything higher than the wall pipe and keep it going "down hill". – JPhi1618 Mar 26 '19 at 17:31
  • Oh, also, the dishwasher drain pipe is clamped up to form a "high loop". You want to keep it strung up like that to prevent drain water from backing up into the dishwasher. – JPhi1618 Mar 26 '19 at 17:33
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As others have pointed out the horizontal section of pipe leading to the trap has to be higher than the piece exiting. This is a code violation and can cause clogs to occur.

The piece of pipe leaving the garburator might also a code violation as it appears not to have the minimum 1/4" per foot of slope, it is also hard to tell if that is a sanitary tee though I assume it is given the threaded connections.

Slope in wdv should be not too much to cause water to flow too quickly leaving solids behind to dry and clog a pipe and not too little to cause solids to sit and not be moved with the liquid.

The problem here seems to be that the sanitary tee that is connecting up with the stack is too high (possibly this was originally plumbed without the garburator).

If I had to fix this, I'll plumb the sink to the trap (fixing the orientation first). Then put in a sanitary tee in the horizontal section and hook in the garburator outflow. I think putting in a sanitary tee into the correct height of the stack is likely too much work.

This is the ideal configuration but if you look at the trap outflow you can see it is lower than the bottom of the garburator:

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  • I have never heard the term "garburator". I looked it up and the Internet says it's used in the Canadian dialect of English. The pipe exiting the... garburator has a downward slope of 3.5 degrees that according to my calculations is above 0.7inches per foot. The perspective of the picture might make this difficult to see. Yeah in my case the drain height is just below the output of the disposal so I do not have all that room to work with. – Alessio Sangalli Mar 26 '19 at 20:10

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